1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a system and method for controlling a state transition of a mobile station having an idle mode.
2. Description of the Related Art
Extensive research into the 4th generation (4G) communication system, which is the next generation communication system, is being conducted to provide users with services that guarantee various qualities-of-service (QoS) at a data rate of about 100 Mbps. In particular, much of the research the 4G communication system is being carried out to support a high-speed service that guarantees mobility and QoS for BWA communication systems such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system. A system based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard (an IEEE 802.16 communication system) is one of the typical BWA communication systems.
The IEEE 802.16 communication system is specified as a communication system employing an Orthogonal Frequency Division Multiplexing (OFDM) scheme and/or an Orthogonal Frequency Division Multiple Access (OFDMA) scheme (hereinafter referred to as an “OFDM/OFDMA communication system”) to support a broadband transmission network for physical channels of the wireless MAN system.
According to IEEE 802.16 standard, if there is no transmission/reception traffic for a preset amount of time, a mobile station (MS) can operate in an idle mode to minimize power consumption. Therefore, if there is no traffic, the MS transmits a De-Registration Request (DREG-REQ) message to a serving base station (BS) with which it is currently communicating, in order to operate in the idle mode. Upon receiving the DREG-REQ message, the serving BS transmits a De-Registration Command (DREG-CMD) message to the MS to approve a transition operation to the idle mode. Formats of the DREG-REQ and DREG-CMD messages are shown in Table 1 and Table 2, respectively.
TABLE 1SyntaxSizeNotesDREG-REQ message format( ) { Management message type=498 bits De-Registration_Request_Code8 bits0x00=MSS De-Registration requestfrom BS and network0x01=request forMS De-Registrationfrom serving BS andinitiation of MS IdleMode0x02-0xFF=Reserved TLV encoded parametersvariable}
As shown in Table 1, in the DREG-REQ message, a Management Message Type field is used for identifying Medium Access Control (MAC) management messages defined in the IEEE 802.16 standard. A De -Registration_Request_Code field is used to indicate the purpose for a MS to transmit the DREG-REQ message. A Type/Length/Value (TLV) encoded parameters field includes parameter information for authenticating MSs based on information of a security association (SA) made between the MSs and the serving BS.
TABLE 2SyntaxSizeDREG-CMD_Message_Format( ) {Management Message Type8bitsAction Code8bitsTLV_Paging Information48bitsTLV_REG-duration24bitsTLV_HMAC Tuple176bits}
As shown in Table 2, in the DREG-CMD message, a Management Message Type field is used for identifying MAC management messages defined in the IEEE 802.16 standard. An Action Code field is used to indicate a purpose of the DREG-CMD message, and its code values related to the idle mode include 0×05, 0×06, and 0×07. Action Code=0×05 indicates that the serving BS approves the idle mode request from the MS. Action Code=0×06 indicates that the serving BS commands the MS to retransmit the DREG-REQ message after a lapse of time indicated by a TLV_REG-duration field. Action Code=0×07 indicates that the serving BS commands the MS to wait until the DREG-CMD message is retransmitted. A TLV_Paging Information field includes Paging Group ID (PG-ID), PAGING_CYCLE and PAGING_OFFSET parameters, all of which are related to the idle mode operation. The PG-ID parameter is assigned by the system according to a location of the MS. That is, the PG-ID parameter represents the current location information of the MS.
The PAGING_CYCLE parameter indicates a paging cycle determined by the system and based on a paging cycle request from the MS. The PAGING_OFFSET parameter is used when the system determines a time at which it can page a particular MS through a frame number of the BS and the PAGING_CYCLE parameter. A TLV_REG-duration field, which is provided for Action Code=0×06, is used for indicating a time at which the MS can retransmit the DREQ-REQ message. A TLV_HMAC Tuple field is an information field used for authenticating the current serving BS based on information on the SA made between MSs and the serving BS.
The DREG-CMD message is generally transmitted from the BS to the MS in response to the DREG-REQ message. However, in order to command the MS to transition to the idle mode, the BS may transmit the DREG-CMD message as an unsolicited message even when it has not received a transition request to the idle mode from the MS through the DREG-REQ message.
Therefore, the MS's transition to the idle mode is performed at the request of either the MS or the BS. Accordingly, the BS, determining that the MS has transitioned to the idle mode, holds for a predetermined time only the minimum connection information required by the MS for transitioning from the idle mode back to an awake mode and deletes the other unnecessary information.
The minimum connection information refers to session information of a MAC layer or its higher layers, and information related to the security and privacy. The deleted information refers to the information indicating the air interface resources of the MAC layer or its lower layers. In other words, the deleted information is connection constituent information necessary for data transmission/reception in the MAC layer or its lower layers, such as a connection identifier that will be described below.
A format of the connection identifier is shown in Table 3.
TABLE 3CIDValueDescriptionInitial Ranging0x0000Used by an SS duringinitial ranging aspart of network entryprocessBasic CID0x0001~mPrimary Managementm+1~2mCIDsTransport CIDs2m+1~0xFEFFand SecondaryManagement CIDsMulticast Polling0xFF00~0xFFFEAn SS may be includedCIDsin one or moremulticast groups forthe purposes ofobtaining bandwidthvia polling.These connectionshave no associatedservice flowBroadcast CID0xFFFFUsed for broadcastinformation that istransmitted on adownlink to all SS
Each of the fields shown in Table 3 will be described below. An Initial Ranging CID field indicates a connection identifier for a Ranging Request (RNG-REQ) message transmitted by an MS to the BS in order to be allocated a Primary CID and a Basic CID, and every MS should be aware of this value of 0×0000. The MS, in an association process with the BS, transmits an RNG-REQ message to inform the BS of its own MAC address. The BS maps the MAC address of the MS and connection identifiers indicating the MS, i.e., a Primary Management CID and a Basic CID, with the MS.
In addition, the connection identifiers include Broadcast CID, Multicast polling CID, Transport CID, Secondary Management CID, etc.
Although the connection identifiers are used for a header of a MAC frame for identification of the connection as can be understood from Table 3, they can also be used for a MAC Service Data Unit (SDU), like the connection identifiers used for a Traffic Indication (TRF-IND) message.
The BS deletes the connection identifier information of the MS that has transitioned to the idle mode. Therefore, in order to resume data communication in an upper session over the MAC layer, an MS in the idle mode must be reallocated the connection identifiers from the BS. To this end, the MS needs to perform a network re-entry process with the BS. In other words, the MS requires the network re-entry process in order to transition from the idle mode to the awake mode, i.e., a normal service state.
FIG. 1 is a signaling diagram illustrating an MS-initiated state transition process to the idle mode in a conventional BWA communication system.
Referring to FIG. 1, if an MS 100 in the awake mode desires to transition to the idle mode in step 111, the MS 100 transmits a DREG-REQ message with De-Registration_Request_Code=‘0×01’ to a BS 150 in step 113. The De-Registration_Request_Code=‘0×01’ indicates that the MS 100 requests transition to the idle mode. At the time of the DREG-REQ message transmission, the MS 100 activates (or starts) a T32 Timer in step 127. The T32 Timer indicates a time for which the MS 100 waits for receipt of a DREG-CMD message from the BS 150 after transmitting the DREG-REQ message. If the T32 Timer expires, the MS 100 retransmits the DREG-REQ message to the BS 150, determining that the BS 150 has failed to receive the DREG-REQ message.
Upon receiving the DREG-REQ message from the MS 100, the BS 150 transmits a DREG-CMD message to the MS 100 in step 115, determining that the MS 100 desires to transition to the idle mode. The BS 150 transmits the DREG-CMD message to the MS 100, to allow the MS 100 to transition to the idle mode. In this case, an Action Code field in the DREG-CMD message is set to ‘0×05’ to indicate approval of the transition request to the idle mode from the MS 100.
Thereafter, after step 115, the BS 150 holds only the minimum information (session information of upper layers and security information) for the MS 100, and deletes all the other connection information related to an air interface of the MAC layer or its lower layers, in step 125. Herein, the BS 150 starts a Management Resource Holding Timer in step 119 and deletes the connection information in step 125 at a time when the Timer expires in step 123, instead of immediately deleting the connection information. This is to hold information on the MS 100 for a predetermined time, in order to process a possible urgent transmission message from the MS 100 or to allow the MS 100 to smoothly re-enter the awake mode without the network re-entry process. The Management Resource Holding Timer is started or reset during transmission of the DREG-CMD message.
Upon receiving the DREG-CMD message from the BS 150, the MS 100 ends the T32 Timer in step 129 and transitions to the idle mode in step 117, thus minimizing power consumption.
FIG. 2 is a signaling diagram illustrating a BS-initiated state transition process to the idle mode in a conventional BWA communication system.
Before a description of FIG. 2 is given, it should be noted that the DREG-CMD message is an unsolicited message used by the BS to command the MS to transition to the idle mode even though there is no separate transition request to the idle mode from the MS.
Referring to FIG. 2, a BS 250 transmits in step 211 a DREG-CMD message to command an MS 200 in the awake mode (in step 223) to transition to the idle mode. At the same time, the BS 250 starts a Management Resource Holding Timer in step 213, and deletes connection information for the MS 200 in step 221 when the Timer expires in step 219. That is, the BS 250 transmits the DREG-CMD message, taking into consideration that the MS 200 will transition to the idle mode.
Upon receiving the DREG-CMD message from the BS 250 in step 211, the MS 200 transitions to the idle mode in step 215.
FIG. 3 is a signaling diagram illustrating a conventional operation performed when an MS fails to receive a DREG-CMD message.
Referring to FIG. 3, if an MS 300 desires to transition from the awake mode to the idle mode in step 311, the MS 300 transmits a DREG-REQ message with De-Registration_Request_Code=‘0×01’ to a BS 350 in step 315. At the time of the DREG-REQ message transmission, the MS 300 starts a T32 Timer in step 313, and then waits until the T32 Timer expires for receipt of a DREG-CMD message from the BS 350.
Upon receiving the DREG-REQ message from the MS 300, the BS 350 transmits in step 319 a DREG-CMD message to the MS 300 in response to the received DREG-REQ message. In this case, an Action Code field in the DREG-CMD message is set to ‘0×05’ to indicate approval of the transition request to the idle mode from the MS 300. While transmitting the DREG-CMD message, the BS 350 starts a Management Resource Holding Timer in step 317, instead of immediately deleting connection information for the MS 300.
However, if the DREG-CMD message transmitted by the BS 350 fails to correctly arrive at the MS 300 in step 319, the T32 Timer expires in step 321. The MS 300 retransmits the DREG-REQ message to the BS 350 in step 323, since the DREG-REQ message transmitted in step 315 has failed to correctly arrive at the BS 350. At this time, the T32 Timer also restarts in step 321.
The BS 350 transmits a DREG-CMD message in step 325 in response to the DREG-REQ message received in step 323. However, if the MS 300 fails to normally receive even the DREG-CMD message transmitted in step 325, the MS 300 and the BS 350 repeat the operations in steps 315 through 323.
In other words, if the MS 300 fails to receive the DREG-CMD message from the BS 350 in response to the DREG-REQ message, the MS 300 repeatedly retransmits the DREG-REQ message each time the T32 Timer expires, as many times as the preset number of retransmissions.
If the number of retransmissions for the DREG-REQ message exceeds the preset number of retransmissions, the MS 300 initializes a MAC state with the BS 350 (MAC re-initialization) in steps 337 and 339, determining that communication with the BS 350 is dropped. Accordingly, the MS 300 performs a network re-entry process with the BS 350 or another BS.
The BS 350, since it never received the DREG-REQ message from the MS 300, deletes all connection information for the MS 300 in step 343 when the Management Resource Holding Timer expires in step 341.
As described with reference to FIG. 3, if an MS fails to normally receive a DREG-CMD message from a BS in response to a DREG-REQ message, the MS performs MAC re-initialization, thereby acquiring state synchronization between the BS and the MS. In other words, if the BS transmits a DREG-CMD message to the MS, the BS expects that the MS will transition to the idle mode. However, the MS, if it fails to receive the DREG-CMD message, will stay in the awake mode, causing state mismatch between the BS and the MS. However, due to the MAC re-initialization operation of the MS in step 339, state synchronization between the BS and the MS is reacquired.
FIG. 4 is a signaling diagram for a description of possible problems occurring when a conventional BS sends an unsolicited state transition request to the idle mode.
Referring to FIG. 4, a BS 450 transmits an unsolicited DREG-CMD message to an MS 400 in step 413 to command the MS 400 in the awake mode (in step 411) to transition to the idle mode. At the same time, the BS 450, as it expects that the MS 400 will transition to the idle mode, starts a Management Resource Holding Timer in step 415 and deletes connection information for the MS 400 in step 421 if the Timer expires in step 419.
However, the MS 400 will continue to hold the awake mode if it fails to receive the DREG-CMD message from the BS 450. Therefore, the BS 450 will delete the connection information for the MS 400 if the started Management Resource Holding Timer expires in step 419. This means that the BS 450 deletes all connection information for the MS 400, making communication between the MS 400 and the BS 450 impossible.
To make the communication between the MS 400 and the BS 450 possible, the MS 400 needs to perform a network re-entry process with the BS 450. However, the MS 400 does not perform the network re-entry process, because the MS 400 is still under the assumption that it should sill operate in the awake mode. Thereafter, if the MS 400 performs data transmission to the BS 450 in the awake mode in step 423, the BS 450 disregards the message transmitted by the MS 400 in step 425 because it has no connection information for the MS 400.
The BS 450 can allocate the deleted connection information, i.e., connection identifier, for the MS 400 to another MS that performs an initialization process to the BS 450. For example, if a connection identifier used by a first MS is ‘1’, the BS 450 can allocate the connection identifier ‘1’ to a second MS as described above, if the BS 450 deletes the connection information for the first MS. Therefore, the first MS and the second MS hold the same connection identifier, and perform data transmission/reception using the connection identifier. As a result, when the first MS transmits data using the connection identifier ‘1’, the BS 450 may mistake the data for one transmitted by the second MS. Also, when the BS 450 transmits data to the second MS, the first MS may mistake the data for data transmitted thereto. Accordingly, there is a need for a scheme for acquiring state synchronization between a BS and an MS transitioning from the awake mode to the idle mode.